[Qemu-devel] qxl screendump: how and why it is broken

[Alon Levy <alevy@redhat.com>]

..and why we got to this situation.

(per Anthony's request, and if you'll excuse the puny attempt to write
biblically, a new beginning)

In the begining there was screendump. It was a synchronous monitor
command. Libvirt said: I want a screendump. And qemu waited for the vga
device to produce it. When the vga device said it was done, libvirt was
done.

Then there came qxl. Qxl used a spice library to render the contents. It
would say to libvirt: wait, while I read on this thread, which will tell
me I am done. And so it was possible to hang the libvirt process:

 tl;dr if spice client hangs libvirt screenshot hangs and qemu's vcpus starve.
 http://lists.gnu.org/archive/html/qemu-devel/2012-03/msg00864.html

Then a new patchset landed, which stopped the read, by waiting for a
callback from the spice server. It is here:
 http://patchwork.ozlabs.org/patch/142985/
 commited as 81fb6f1504fb9ef71f2382f44af34756668296e8

But now screendump for qxl is broken: It produces a file that is old,
because it calls ppm_save before spice server is done rendering.

Now we come to now. My understanding from the previous thread is:
 * monitor async is not good because Luiz said so.
 (yes, I know I'm putting my head in the sand here)

If the above is enough to convince you of $SUBJECT, then the
screendump-async patchset is still waiting review :)

http://patchwork.ozlabs.org/patch/144704/
http://patchwork.ozlabs.org/patch/144705/
http://patchwork.ozlabs.org/patch/144706/

Alon